1. Field of the Invention
The present invention relates to a micro electromechanical switch for opening and closing an electronic circuit by causing contact or separation between contacts using electrostatic attraction, a method for manufacture of such, and a device utilizing the micro electromechanical switch. In particular, the present invention relates to a structure of an actuator of a micro electromechanical switch.
2. Background Art
A conventional micro electromechanical relay, which is one type of a micro electromechanical switch, will be explained with reference to FIGS. 40-45. FIG. 40 shows schematically the conventional micro electromechanical relay. The micro electromechanical relay 100 comprises a base 101 and an actuator 111 having a portion thereof fixed to an upper face of the base 101 and also having the other portion separated from the base 101. Furthermore, within these figures, an element that is the same is designated by the same reference.
A fixed electrode 102 and a pair of signal lines 103 and 104 are disposed on the upper face of the base 101. The pair of signal lines 103 and 104 is aligned at a short distance. The opposing parts of the signal lines 103 and 104 form a pair of fixed contacts 103a and 104a, respectively.
The actuator 111 comprises a supporting portion 112, a beam portion 113, a movable electrode 114, and a movable contact portion 115. The supporting portion 112 is disposed on a surface of the base 101, extends upward therefrom, and supports the beam portion 113, the movable electrode 114, and the movable contact portion 115. The beam portion 113 extends from the supporting portion 112 as a cantilever beam for elastically supporting the movable contact portion 115 as well as for elastically supporting the movable electrode 114 through a connecting part 118. The movable contact portion 115 is disposed at a distal tip of the beam portion 113, and the movable electrodes 114 are disposed on both sides of the beam portion 113 through the connecting part 118. The connecting part 118, the beam portion 113, and the movable electrode 114 all have the same thickness.
The movable electrode 114 is disposed at a position opposite to the fixed electrode 102 of the base 101. Furthermore, an insulating film 110 is formed on the fixed electrode 112 for prevention of short circuiting between the fixed electrode 102 and the movable electrode 114. The movable contact portion 115 is disposed at a position opposing a region extending from the fixed contact 103a to the fixed contact 104a, and a movable contact 116 is disposed at a lower face of the movable contact portion 115. The movable contact 116 opposes each of the fixed contacts 103a and 104a and provides mutual electrical contact between the signal lines 103 and 104 by closing contact between the fixed contacts 103a and 104a. 
FIGS. 41(a) and (b) show a state when a voltage is not applied between the fixed electrode 102 and the movable electrode 114. As shown by these figures, the fixed electrodes 103a and 104a are displaced from the movable contact 116, and the signal line 103 and the signal line 104 are mutually electrically separated.
FIGS. 42(a) and (b) show a state when a voltage is applied between the fixed electrode 102 and the movable electrode 114. As shown by these figures, the movable electrode 114 is driven toward the fixed electrode 102 by electrostatic attraction generated by the applied voltage. By way of this, the movable contact 116 comes into contact with the fixed contacts 103a and 104a, and the signal lines 103 and 104 become mutually electrically connected. In this state, the contact force required for stabilizing contact resistance between the movable contact 116 and the fixed contacts 103a and 104a needs to be imparted to the movable contact portion 115 by the electrostatic attraction.
When the voltage between the fixed electrode 102 and the movable electrode 114 stops, the electrostatic attraction disappears, and the actuator 111 returns to the original position, as shown in FIGS. 41(a) and (b), due to restorative force of the beam portion 113 and the movable electrode 114. At this time, a restorative force greater than the contact force between the movable contact 116 and the fixed contacts 103a and 104a needs to be imparted to the movable contact portion 115. This restorative force is determined by the elastic constant of the beam portion 113, the elastic constant of the contact part 118, and the inter-contact distance between the movable contact 115 and the fixed contacts 103a and 104a. 
Operation of the movable electrode due to application of voltage is explained with reference to FIGS. 43 and 44. FIG. 43 shows the relevant components of the conventional micro electromechanical relay 100 shown in FIG. 40. Moreover, FIGS. 44(a)-(d) are cross-sectional diagrams, along the R-R line shown in FIG. 43 from the movable electrode 114 to the movable contact portion 115, showing movement of the movable electrode 114 due to electrostatic attraction.
The conventional movable electrode 114 operates in the below described manner. Specifically, when a voltage is not applied, the movable electrode 114 is disposed as shown in FIG. 44(a). Then, when a voltage is applied, firstly as shown in FIG. 44(b), the outer side of the movable electrode 114 is deformed toward the fixed electrode 102 due to electrostatic attraction. The electrostatic attraction between the electrodes (Fele) is expressed by the below listed equation:Fele=(C×Vs2)/(2×d)  (11)Where C is the electrical capacitance, Vs is the applied voltage, and d is the inter-electrode distance.
Due to deformation of the movable electrode 114, the distance between the movable electrode 114 and the fixed electrode 102 becomes smaller, and thus the electrostatic attraction according to the equation (11) becomes larger. Accordingly, as shown in FIG. 44(c), the movable electrode 114 and the movable contact portion 115 move toward the base 101.
Due to movement of the movable electrode part 114 toward the base 101, the distance between the movable electrode 114 and the fixed electrode 102 becomes smaller, and the electrostatic attraction according to the equation (11) increases further. Thus, as shown in FIG. 44(d), the movable electrode 114 and the movable contact portion 115 moves further toward the base 101, and thereby the movable contact 116 comes into contact with the fixed contact 103a. 
The amount of displacement of the actuator 111 due to application of voltage will be explained while referring to FIG. 45. FIG. 45 shows the results of a simulation of the amount of displacement when a voltage is applied to the conventional actuator 111. When points of equal amount of displacement are interconnected by contour lines, the amount of displacement is indicated by densities of dots within regions bounded by the contour lines and the profile of the movable electrode 114. Namely, the region without dots indicates the region of near zero amount of displacement, and the region of highest density of dots indicates the region of contact between the movable electrode 114 and the fixed electrode 102.
Referring to FIG. 45, for the conventional movable electrode 114, it may be understood that the amount of displacement is small, and there is no adherence to most portions of the fixed electrode.    [Patent citation 1] Unexamined Laid-open Patent Application H11-111146 (disclosed on Apr. 23, 1999)    [Patent citation 2] Unexamined Laid-open Patent Application H11-134998 (disclosed on May 21, 1999)
As discussed above, sufficient contact force and restorative force are required in order for the micro electromechanical relay 100 to operate normally. The voltage applied between the fixed electrode 102 and the movable electrode 114 may be increased in order to raise the contact force by increasing the electrostatic attraction. The below listed 3 methods have been considered for increasing the electrostatic attraction:    (Method A): The elastic constant is decreased by reduction of thickness of the beam portion 113 and the movable electrode 114, without changing the shapes of the beam portion 113 and the movable electrode 114 as viewed from above, and also the distance between the fixed electrode 102 and the movable electrode 114 at the time of application of voltage is decreased as much as possible.    (Method B): The applied voltage is raised.    (Method C): The dimensions of the fixed electrode 102 and the movable electrode 114 are increased.
However, when the elastic constant is decreased by method A, the restorative force also decreases. Thus, there would be concern that contact between the movable contact 116 and the fixed contacts 103a and 104a may continue even after stoppage of the application of voltage. Moreover, the methods B and C run counter to trends of technical progress toward lower voltage and further miniaturization.
In view of above, the present invention has an object of providing a micro electromechanical switch capable of improving the contact force while maintaining the restorative force, lowering the applied voltage, and/or decreasing dimensions of the electrode.